Not applicable.
Not applicable.
This invention relates to the field of microscopy, and in particular, to measuring motions of microscopic structures.
There are many examples of microscopic structures with moving parts. A broad class of man-made structures is called micro-electromechanical systems (MEMS), which includes microelectro-optical systems (MOEMS). MEMS offer possibilities for the development of microscopic sensors, electro-optical components, and mechanical actuators. Often MEMS involve the motion of one or more parts. These motions can be as small as several nanometers and as fast as hundreds of megahertz.
Prior art methods for measurements of microscopic structures include laser vibrometers, white light interferometers, laser triangulation, video microscopy, and computer microvision.
The class of measurement systems that includes laser vibrometers provide high speed motion estimates. Laser vibrometers are typically capable of measuring motions of a reflective structure at one or a plurality of locations (for example an array of locations). Laser vibrometers typically project a spot of laser light on a target. A sensor measures changes in brightness due to interference between the projected light and light reflected by the target. Laser vibrometers can measure motions at hundreds of megahertz.
The class of measurement systems that includes white light interferometers (sometimes referred to as profilometers) typically measure variations in the height (out-of-plane) of structures. Structures with heights on the order of a nanometer can be measured.
The class of measurement systems that includes video microscopy typically measure both static and dynamic structures. Computer microvision refers to systems that combine an off-the-shelf light microscope, an off-the-shelf CCD camera, and stroboscopic illumination to capture images of small moving structures and machine vision algorithms to analyze those images. Small motions on the order of nanometers (much smaller than the resolution of the objective lens) can be resolved in three dimensions. Motions of almost any structure in the image can be measured. Measurable structures are structures that provide a spatial change in brightness or contrast that can be exploited by the machine vision algorithms.
A technique typically used by computer microvision systems is stroboscopic illumination. Stroboscopic illumination is a way of replacing a fast frame-rate camera with a fast and bright light source. Periodic motions, such as the vibrating tines of a tuning fork, are well suited to imaging using stroboscopic illumination. For example, a tuning fork may vibrate back and forth six thousand times a second. This is much faster than the frame rate of a video camera (30 frames/second). Under continuous lighting, the tines appear as a blur. However, a strobed light source can produce very short pulses of light that allow still frame images to be taken of the moving fork. Using a sequence of such images, the motion of the tuning fork can be reconstructed. The application of this technique to interferometry is particularly useful for out-of-plane motions.
It is known to use stroboscopic illumination to provide repeated illumination of a repetitive motion at the same position by light pulses synchronized with the motion and the duration of which is short relative to the speed of the motion desired to be captured. By this method, an image of a fast moving structure may be acquired by integrating the light reflected from numerous images of the structure in the same relative position. A sequence of images can be acquired in this manner such that a complete period of motion can be reconstructed. Previous implementations of stroboscopic illumination have used light pulse pairs to determine the size and velocity of a moving particle (Labrum, et al, U.S. Pat. No. 4,136,950). The motion of the particle and the strobe are not synchronous. However, a drive signal may be used to produce periodic motions of a microscopic structure while the illumination source is turned on and off in accordance with a trigger signal synchronous to the drive signal. As the structure moves, the phase of its motion repeats. An image of its position at any phase of its motion can be acquired by illuminating the structure at precisely the phase of interest. The duration of the pulse is short with respect to the speed of motion of the structure such that the resulting image is not blurred by the motion of the structure. A sequence of images acquired at different phases of motion can be used to represent a complete period of motion.
Machine vision algorithms represent a class of image processing algorithms. Given a time sequence of digitized images of a moving object, machine vision algorithms can be used to estimate the motion of the object. In particular, optical flow algorithms are a category of machine vision algorithms that have been used in previous computer microvision implementations.
Optical flow algorithms can be used to compare the spatial gradients in brightness within an image and the temporal gradients in brightness across a sequence of images. These variations in brightness are used to estimate motion. Other algorithms exist which use the brightness data across a sequence of images in a manner different from optical flow. For example, the centroid of brightness can be calculated, and its position can be compared across a sequence of images to estimate motion.
It is an important object of the invention to provide improved methods and means for measuring motion of microscopic structures.
The invention, in one aspect, provides an integrated system and a method to measure motions (with up to six degrees of freedom) of microscopic structures, for example MEMS. In one embodiment, the apparatus comprises a computer server, an optics module, a video display for displaying images captured in real-time, an electronics module including a CCD camera, a mechanical mount and stage, and a custom software package including a Web-based user interface and algorithms for image analysis.
A CCD camera acquires images from the optics module. The electronics module transfers signals representative of these images to the server. The integration of a field programmable gate array (FPGA) architecture and the CCD camera provide useful visualization schemes, such as slow motion, in which selected phases of motion may be illuminated by a strobed light source, and images displayed. The result is to slow the apparent motion of the structure. For example, a structure moving at 100 kHz could be viewed such that the apparent motion was 1 Hz. Stop action video may also be provided by illuminating a single phase of motion by the strobed light source. Real-time video imagery may be acquired and displayed. The result is to stop the apparent motion of the structure.
In another aspect, the invention has an adjustable stage with up to six degrees of freedom, which may be controlled by the computer.
In yet another aspect, the invention has provision for a shuttered reference light path to be used to provide for interferometric measurements, which may be used to measure out-of-plane motions.
In still another aspect, the motion of the target may be controlled by a signal determined by the computer and produced by a field-programmable gate array. This signal may be synchronized with an external signal through the use of a phase-locked loop.
Multiple laser light sources may also be used to provide illumination (both brightfield and darkfield) and interferometric measurements at different wavelengths. Kohler illumination may also be used.
The computer may be controlled over a network. Network access to the system provides a number of advantages. A single instrument, centrally located, can be used remotely by a larger community of individuals than could use a typical instrument. Remote individuals access the same functionality as individuals manipulating the system directly. For these and similar reasons, the instrument is well suited to production environments such as clean rooms.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will become apparent from the description, the drawings, and the claims.